1. Field of the Invention
The present invention relates to a projection display device used in an image processing field, and more specifically, the present invention relates to a projection display device in which images of red, green and blue color lights formed by display elements such as liquid crystal panels are projected onto a screen by projection lenses and these images are superposed on the screen.
2. Description of the Related Art
In a prior art projection display device for obtaining a large color display, three cathode lay tubes (CRT) and three projection lenses are used to project images of red, green and blue color lights onto a screen so as to superpose these images on the screen. However, the projection display device using cathode lay tubes is very large in size, and it is not actually used.
Recently, projection display devices using liquid crystal panels have been proposed. For example, Japanese Unexamined Patent Publication (Kokai) No. 2-253292 and No. 3-44623 disclose a projection display device comprising a source of light, dichroic mirrors for separating the light from the source into blue, green and red color light, three liquid crystal panels forming images of red, green and blue color lights, respectively, a color mixing optical element(s) such as a dichroic mirror or a dichroic prism for fixing the images formed by the liquid crystal panels, and a projection lens for projecting the mixed images to a screen.
Also, there is another known projection display device disclosed, for example, on pages 201 to 203 of the publication entitled "Color Liquid Crystal Display", published in 1990 in Japan. This projection display device comprises a source of light, dichroic mirrors for separating the light from the source into red, green and blue, three liquid crystal panels forming images of blue, green and red color light, respectively, and three projection lenses arranged in association with the liquid crystal panel to form sets individually. The images of red, green and blue color lights are projected onto a screen by projection lenses and these images are superposed on the screen. This projection display device has a more compact optical system than that of the above described projection display device including the color mixing optical element, and the device as a whole should be made small. Accordingly, in order to obtain a compact device, it is desirable to arrange the device so that three liquid crystal panels and three projection lenses are used for projecting the images of blue, green and red color light onto the screen so as to superpose the images on the screen. Also, in this publication, mirrors are arranged between the liquid crystal panels and the projection lenses, respectively, so that all the liquid crystal panels are arranged in a common vertical plane and the projection lenses are arranged in a horizontal plane.
In the arrangement using three liquid crystal panels and three projection lenses, three projection lenses are arranged in a row in a side by side relationship on a line extending horizontally and parallel to the screen with respective optical axes extending parallel to each other toward the screen. Accordingly, the axis of the central projection lens is directed to the center of the screen, and the axes of the outer projection lenses are directed to positions slightly shifted from the center of the screen. If the axes of all the projection lenses are directed to the center of the screen, the optical axes of the outer projection lenses are inclined relative to the screen, so that the image projected from the outer projection lens is distorted to a trapezoidal shape and does not coincide with the images projected from the remaining projection lenses. Accordingly, the resultant image may be distorted.
To superpose the images on the screen, the centers of the images projected from all of the projection lenses must coincide with the center of the screen. To this end, the axes of the outer liquid crystal panels are preferably arranged at positions slightly shifted from the axes of the associated outer projection lenses, respectively, so that a light obliquely incident on the center of the outer liquid crystal panel and passing through the center of the projection lens reaches the center of the screen. Hereinafter, this is referred to an offset of axis. If the amount of the offset of axis becomes large, a problem occurs in that uneven color distribution and color shift tend to increase because of an unbalanced mixing of red, green and blue color lights and a directional distribution of light intensity occurring on the screen.
Concurring recent requirements for a high definition display (an increase in the number of pixel elements), the size of liquid crystal panels are increasing. In the case where all of the liquid crystal display panels are arranged on a common vertical plane (and all of the liquid crystal display panels face the screen), the distance between the centers of the adjacent liquid crystal display panels is the width of one liquid crystal display panel plus a space between the adjacent liquid crystal display panels. The distance between the centers of the adjacent liquid crystal display panels is smallest when the adjacent liquid crystal display panel are brought into contact. In the prior art, the liquid crystal display panels are substantially coaxially arranged with the projection lenses, respectively, and the distance between the centers of the adjacent projection lenses substantially equals the distance between the centers of the adjacent liquid crystal display panels. When the mirrors are arranged in the prior art, as described in the above referenced publication, the distance between the centers of the adjacent projection lenses substantially equals the distance between the centers of the adjacent liquid crystal display panels, because all of the liquid crystal panels are arranged on a common plane and all of the projection lenses are also arranged on a common plane.
Therefore, the distance between the centers of adjacent projection lenses becomes larger with an increase in the size of the liquid crystal panels. When the distance between the centers of the central projection lens and the outer projection lens becomes larger, the inclination of the line passing through the center of the outer projection lens and the center of the screen becomes larger and the amount of the offset of axis also becomes larger. If the amount of the offset of axis becomes larger, problems arise in that the resolution and brightness of the image at the peripheral region thereof are reduced, the viewing angle dependency of the liquid crystal increases, and the contrast is reduced. For this reason, there are severe requirements for the projection lenses. Also, a problem arises in that uneven color distribution and the color shift become larger since a convergent angle becomes larger.
It is preferable to constitute the screen from a plate having lenticules on opposite surfaces thereof in order to prevent uneven color distribution and color shift. While the screen made by the plate having lenticules on opposite surfaces thereof is effective for preventing uneven color distribution and color shift, resolution of the display is relatively low because it is difficult to manufacture very thin plate having lenticules on opposite surfaces thereof with an accurate pitch. It is preferable to constitute the screen from a plate having a lenticule on one surface thereof and a light diffusion plate in order to improve the resolution of the display, but it is difficult to reduce the uneven color distribution and the color shift when the distance between adjacent projection lenses is relatively large.